A transom scupper drain is almost a necessity for most boats today. This is centered around a device that seals an opening in the bottom of the transom, usually below the actual waterline. This must have a lid on the outside that is normally closed to prevent the entrance of seawater, but that can open, or be opened, to allow the bilge water, from leakage, rain, or spray, to be drained when the boat is on land, or in use at a speed through the water that is enough to clear the transom.
In most situations, it should be self-opening when the pressure of the water on the inside, from the bilge, is greater than the pressure of the water on the outside. This, again, happens when the boat is out of water, or moving through the water fast enough to clear the transom.
There are very many devices--too many to be listed here--that can be used in combination with a hole in the bottom of the transom, with a valve of some kind, that can be opened to let the water drain out. There are many variations of this with varying degrees of simplicity and effectiveness. Some are manually controlled, but for the type considered here, again, the stopper or lid should function automatically to seal itself against water trying to come in from the outside, but open itself to allow water to flow out of the hull when the pressure from the inside is greater than the pressure from the outside.
For this type of scupper drain valve, a lid must cover the opening, and must be held in place to prevent sea water from pouring into the bilge. The most common versions have a spring of some kind urging the lid or cap against the base of the device that covers the opening in the bottom of the transom. Under ideal conditions, when the spring is new, this spring device can function quite well, but with age and wear, and deterioration of the metal of the spring, the lid may not open in a timely manner, or adequately, to allow the bilge to drain out, or may not close properly to keep the sea water from coming back in.
Metallic springs are, inevitably, subject to corrosion, and mechanical deterioration--particularly when in contact with sea water. Plastic springs, on the other hand, deteriorate when exposed to heat and sunlight, as well as from ageing, and in other ways.
The object of this invention is to provide a lid closure that has a consistant, very-light pressure that is just enough to hold the lid closed when there is no other pressure on the lid, yet openable at minimal pressure from the inside to let the bilge water out. Sea water from the outside will meet a closed lid, and the pressure from the outside will seal the lid more tightly.
The very-light pressure on the closure should be consistent at all times, and should not vary from season to season, or over a considerable length of time. The mechanism should be easy to maintain, repair, or replace.
This is accomplished here by a gravity-controlled, hinged lid that rests on and covers a tubular opening that may be sloped well away from the vertical. The tubular opening and hinged lid are part of a unit that can be mounted in a watertight manner over a hole in the transom of a boat below the level of the bilge. Since this is controlled by gravity, rather than a spring, it will function consistently and uniformly from season to season, year in and year out, almost indefinitely.